1. Field
The present disclosure relates to a method and a composition for depolymerization of cured epoxy resin materials using transition metal salts. More particularly, the present disclosure relates to a method for depolymerization of cured epoxy resin materials using transition metal salts and a composition used therefor, a method for separating fillers from cured epoxy resin materials, fillers obtained by the method, etc.
2. Description of the Related Art
An epoxy resin is a thermosetting resin including a network polymer formed by ring-opening of epoxy groups generated when mixing an epoxy monomer having two or more epoxy groups in its molecule with a curing agent. Such an epoxy resin has excellent chemical resistance and durability, shows a low volumetric shrinkage upon curing, and thus may be used as a highly functional raw materials which are essential for various industrial fields, including adhesives, paints, electronic/electric industry, civil engineering/architectural industry, or the like.
In the field of composite materials that have been spotlighted recently, an epoxy resin is combined with various filler materials and used for various applications, including space aviation, info-communications and new energy fields. Thus, there has been an increasing demand for such epoxy resins. Particularly, carbon fiber reinforced plastics (CFRP) obtained by combination with carbon fibers are light and show excellent physical properties and durability, and thus are used widely as a key material in the field of cars or space aviation. In addition, epoxy resins combined with the other polymer resin materials have been used widely for producing high-performance materials.
An epoxy resin, once cured, forms a three-dimensional crosslinked network structure, which has strong resistance against chemicals. Thus, such an epoxy resin advantageously imparts materials with high durability and anticorrosive property, but undesirably has a difficulty in treating and reutilizing the materials after use.
Most cured epoxy resin materials have been treated by reclamation. This is significant waste in terms of cost and may cause severe environmental pollution.
Recently, a composite material of an epoxy resin with a filler has been increasingly in use, active studies have been conducted about methods for decomposing an epoxy resin selectively so that a filler material more expensive than an epoxy resin may be separated effectively.
Currently, the most generally used method for decomposing an epoxy resin and separating a filler material is a pyrolysis process. For example, in case of carbon fiber reinforced plastics (CFRP) using carbon fibers as fillers, Japanese companies, such as Toray, Teijin and Mitsubishi, have been taking the lead in treatment and recycle of CFRP based on pyrolysis in an amount of 1000 tons per year. However, separation of carbon fibers through a pyrolysis process requires a pretreatment process including preliminarily crushing CFRP mechanically. When CFRP is crushed to a size of several millimeters, carbon fibers may also be crushed and the carbon fibers to be recycled have a decreased length, which adversely affect the properties of carbon fibers. In addition, above all, such a pyrolysis process requires a high temperature of 500° C. or higher where organic compounds produce materials, such as dioxin, harmful to the human body due to combustion thereof.
Therefore, various chemical decomposition processes have been studied to decompose an epoxy resin efficiently at a lower temperature.
For example, when an epoxy resin is decomposed under supercritical or subcritical fluid, it is possible to treat a cured epoxy resin material at a temperature of 250-400° C. that is lower than the temperature of a pyrolysis process. When using such a process, there is an advantage in that the recovered filler material is less deteriorated as compared to a pyrolysis process.
However, according to the researches by the inventors of the present disclosure, the above method still requires a high temperature and high pressure of 10 atm or higher. Thus, the above method requires a special processing system capable of resisting such conditions, resulting in poor cost efficiency.
Meanwhile, some studies have been conducted to carry out decomposition of an epoxy resin under milder and more general processing conditions. However, in this case, it is not possible to dissolve various cured epoxy resin materials, including cured multi-functional epoxy resin materials or cured epoxy resin materials using an acid anhydride-based curing agent or aromatic diamine-based curing agent. In addition, a long reaction time and high reaction energy are still required. Moreover, it is problematic that various organic solvents harmful to the human body are used as main solvents for the reaction system.
Particularly, many numbers of conventional methods using an organic solvent as their main solvent for decomposition of an epoxy resin are known.
For example, there is disclosed a method for depolymerization of a waste printed circuit board by introducing an electrolyte containing an alkali metal and adding an organic solvent thereto (Korean Patent Application Publication No. 2011-0113428).
In addition, there is disclosed a method for treating an epoxy resin by crushing and acid-treating the epoxy resin and further treating the epoxy resin with an organic solvent and oxidant in a sealed reaction container (Chinese Patent No. 102391543).
There is also disclosed a method for decomposing an epoxy resin by using hydrogen peroxide and acetone (Green Chem., 2012, 14, 3260).
There is also disclosed a method for treating an epoxy resin composite material with a treatment solution containing a dewatered alkali metal compound and an organic solvent (Japanese Patent Application Publication No. 2005-255899).
It is also disclosed that an epoxy resin prepolymer is allowed to be in contact with an aprotic organic solvent having a dipolar moment of 3.0 or more so that the epoxy resin prepolymer may be dissolved in the solvent (Japanese Patent Application Publication No. 2013-107973).
Further, there is also disclosed use of an organic solvent including furan-2-carbaldehyde as extraction solvent or cracking solvent for separating carbon fibers from CFRP (US Patent Application Publication No. 2014-0023581).
The methods according to the related art disclose that decomposition of epoxy may be carried out by using an organic solvent under a mild condition at a relatively low temperature.
However, the methods according to the related art are using organic solvent reaction systems which adopt an organic solvent itself as their main key for dissolving an epoxy resin. Herein, since the organic solvent itself functions as contaminant source, such organic solvent reaction systems have a fundamental limit in that a problem of pollution caused by the organic solvent should be solved. In addition, such methods according to the related art have low applicability to sparingly decomposable epoxy resins or require a large amount of energy, resulting in unsatisfactory reaction efficiency.